Image forming apparatus responsive to environmental conditions

ABSTRACT

An image forming apparatus responsive to environmental conditions has an image forming device for forming a dot image on a recording member. The image forming device includes exposure apparatus for exposing the recording member. A detector detects a humidity condition of the apparatus, and control circuitry regulates the amount of light of the exposure apparatus in accordance with an output of the detector. The control circuitry is adapted, when the humidity detected by the detector is lower than a predetermined value, to select an amount of light of the exposure apparatus larger than that in the case where the humidity is higher than the predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for forming an image on arecording member.

2. Related Background Art

There is already known an apparatus for forming an image, through anelectrophotographic process, by charging and exposing a photosensitivemember to form an electrostatic latent image thereon and developing saidlatent image.

The sensitivity of a photosensitive member varies with the lapse of timeor by a change in the environmental conditions such as temperature andhumidity. In an apparatus for forming a dot-pattern image on aphotosensitive member by means of a laser beam or the like, there isoften employed a method of modulating each pixel with a predeterminedarea rate. However, in such method, control has been accomplished withpredetermined area rates for the start and stop of laser beam emission,since the points of said start and stop are inevitably distant in time.It is that known time-dependent changes occur in the potential V_(OO) ofthe photosensitive member at the start of laser beam emission and thepotential V_(FF) at the end of laser beam emission, as shown in FIG. 13.

FIG. 13 shows the surface potential as a function of the grid voltage VGof the primary charger. Since the surface potential V_(OO) varies from Ato B in time while the surface potential V_(FF) varies from C to D, itis necessary to vary the grid voltage from 700 V to 1000 V in order toattain a same value of V_(c) =(V_(OO) -V_(FF)) =420 V. However it hasbeen difficult to precisely cover a voltage range from 200 V to 1000 Vin a high-voltage unit.

Also the performance of the developing unit varies with the ambientconditions. Particularly the image density is affected by the humidity,and it is often not possible to obtain an optimum density.

SUMMARY OF THE INVENTION

In consideration of the foregoing, an object of the present invention isto provide an improved image forming apparatus.

Another object of the present invention is to provide an image formingapparatus capable of forming an image with satisfactory tonal renditionregardless of changes in the environmental conditions.

Still another object of the present invention is to provide an imageforming apparatus capable of forming an image with satisfactory tonalrendition regardless of time-dependent changes in the characteristics ofthe recording member.

Still another object of the present invention is to provide an imageforming apparatus capable of forming a color image with constant colorreproduction.

The foregoing and still other objects of the present invention willbecome fully apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams of a color copying machine embodyingthe present invention;

FIG. 2 is a block diagram of a tone control circuit;

FIG. 3 is a timing chart showing signals in a synchronization controlblock;

FIG. 4 is a flow chart of the control sequence of a control unit in areader unit;

FIG. 5 is a flow chart of the control sequence of a control unit in aprinter unit;

FIG. 6 is a flow chart of the control sequence for data output of apattern generator and potential reading;

FIG. 7 is a chart showing the relation between output data of thepattern generator and the potential of a photosensitive member;

FIG. 8 is a cross-sectional view of a color copying machine;

FIG. 9 is a chart showing the relation between print image density andhumidity under a same image forming condition;

FIG. 10 is a chart showing the relation between print image density andsurface potential of a photosensitive member under a same image formingcondition;

FIG. 11 is a chart showing the relation, stored in a ROM in advance,between density data and data measured with a potential sensor;

FIG. 12 is a chart showing the relation between input image signal andoutput image density;

FIG. 13 is a chart showing the relation between the surface potential ofa photosensitive member and control voltage, also indicatingtime-dependent changes in potential;

FIG. 14-1 is a chart showing the relation between electric powersupplied to a laser and light emission therefrom;

FIG. 14-2 is a chart showing the relation between the duration of abinary pulse signal and the intensity of light emission from a laser;

FIG. 15 is a circuit diagram of a laser driver circuit 22;

FIG. 16 is a block diagram of a binary digitizing circuit 44; and

FIG. 17 is a chart showing characteristics of a humidity sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now the present invention will be clarified in detail by embodimentsthereof shown in the attached drawings.

EXPLANATION OF A BLOCK DIAGRAM OF A COLOR COPYING MACHINE (FIG. 1)

FIG. 1 is a block diagram of a color copying machine embodying thepresent invention.

A synchronization signal processing unit 1 generates various timingsignals in synchronization with a horizontal synchronization signal 32supplied from a tone control circuit 21 in response to a signal from abeam detector 20 of a printer unit 200. A contact-type CCD sensor block2 reads and converts an original image into an electrical signal 5, inresponse to a reader horizontal synchronization signal RHSYNC and adrive signal 4 generated by the synchronization signal processingunit 1. A signal processing unit 3 is provided for signal shaping inorder to prevent attenuation of high-frequency components of the signal5.

In an image processing block 6, the image signal from the signalprocessing unit 3 is at first supplied to an analog processing unit 7.As the contact-type CCD sensor block 2 time-sequentially releasessignals of cyan (C), green (G) and yellow (Y) constituting each pixel,the analog processing unit 7 at first separates said signals intorespective colors of C, G and Y. Since a printer unit 200 is providedwith developing stations for yellow (Y), magenta (M) and cyan (C), saidimage signals are converted into red signals (R), green signals (G) andblue signals (B) by calculations C-G=B and Y-G=R. Said R, G and Bsignals thus obtained, linearly varying in voltage in relation to theimage density, are converted into 8-bit digital density signal by an A/Dconverter. These processes are conducted in the analog processing unit7.

The image signal of each color digitized in the analog processing unit 7is divided into 5 channels which are mutually unsynchronized. Thus, saidchannels are synthesized by a jointing memory 8 to obtain unified imagedata. The image data synthesized and converted into Y, M and C signalsin the jointing memory 8 are supplied, in synchronous manner, to animage processing unit (IPU) 9 for effecting a shading correction and amasking correction. Then a desired color signal is selected by a controlunit 10 of the reader unit 100, and an 8-bit color signal afterpredetermined color conversion is supplied from the IPU 9 to the printerunit 200 through a data line 11.

Separately the control unit 10 activates a motor driver 13 to control amotor 12 for scanning the original image, and also controls a CVR unit15 for controlling an exposure lamp 14 and an operation unit 16 forproviding copy instructions and other operations.

There is also provided an unrepresented mode selector switch forachieving a sharp reproduction from a letter original and a tonalreproduction from a photograph original, and the information of suchmode is supplied from the operation unit 16 to the control unit 10 andthen to the printer unit.

In response to said information, the control unit of the printercontrols a selector of a binary digitizing circuit to be explainedlater, according to the signal from a CPU 25-1.

The image data 11 from the reader unit 100 are supplied to a tonecontrol circuit 21 of the printer unit 200. The tone control circuit 21has a function of synchronizing the image clock signal of the readerunit 100 with the image clock signal of the printer unit 200, and afunction of correlating the image data with the reproduced color densityin the printer unit 200. The output signal from the tone control circuit21 is supplied to a laser driver 22 for driving a laser element 23,thereby effecting image formation.

The control unit 25 of the printer communicates with the reader unit 100through a communication control line 24 and controls various units ofthe printer 200. There also are provided a potential sensor 26 fordetecting the charge on a photosensitive member 29, and a potentialmeasuring unit 27 for converting the output signal of the potentialsensor 26 into a digital signal for supply to the control unit 25. Apotential signal supplied to the control unit 25 is fetched by the CPU25-1 thereof for use in a control operation to be explained later. Alsoan image top signal ITOP indicating the leading end of the image from asensor 28 is also supplied to the control unit 25 for controlling therecording operation. Also, signals from a humidity sensor 98 and atemperature sensor 99, for correcting the developing characteristics,are supplied through an A/D converter 25-3 of the control unit 25. Thehumidity sensor 98 in the present embodiment varies its resistanceaccording to the relative humidity, as shown in FIG. 17, showing theresistance in ordinate and the relative humidity in abscissa. Thus therelative humidity ΛH, indicating the ratio of the amount of vapor in theair to the saturated amount of vapor at each temperature is given by:

    ΛH=f(T, H)

wherein T is temperature and H is the indication of the humidity sensor.Said function f is generally represented by a third-order function. Thusthe relative humidity is determined by obtaining said T and H from theoutput signals of the temperature sensor 99 and the humidity sensor 98,converting said signals into digital signals by the A/D converter 25-3of the control unit and processing the thus obtained digital signals.

The relative humidity thus determined is used in a control operation tobe explained later.

FIG. 8 is a cross-sectional view of a copying machine utilizing thecontact-type CCD sensor of the present embodiment.

The copying machine 80 is composed of a reader unit 100 and a printerunit 200. An original scanning unit 83 is adapted to move in a directionA for reading the image of an original document 84 placed on an originalsupport plate, simultaneously turning on an exposure lamp 85 in thescanning unit 83. The light reflected from the original is guided to aconverging rod lens array 86 and focused on a contact-type color CCDsensor 87, consisting of a staggered arrangement of 5 CCD chips of 1024pixels each, wherein each pixel has a size of 62.5 μm (1/16 mm) and isdivided into three areas of 15.5×62.5 μm each, having respectively cyan,green and yellow filters.

The optical image focused on the color CCD sensor 87 is converted intoelectric signals of respective colors, which are subjected to a processto be explained later in an image processing block 88. Thecolor-separated image signals released from said block 88 are suppliedto the printer 200 for image printing.

The color image signals from the reader 100 are used, after pulse widthmodulation etc., for driving the laser element. The laser beam modulatedaccording to the image signals is deflected into a scanning motion by apolygon mirror 89 rotated at a high speed, then reflected by a mirror 90and irradiates the surface of a photosensitive drum 91 to effect dotexposure thereon corresponding to the image. A horizontal scanning lineof the laser beam corresponds to a horizontal scanning line of the imageand has a width of 1/16 mm in the present embodiment. As thephotosensitive drum 91 is rotated in a direction indicated by an arrowat a constant speed, a two-dimensional image is exposed thereon by amain scanning achieved by the movement of said laser beam and a subscanning achieved by said rotation of the photosensitive drum 91. Thephotosensitive drum 91 is uniformly charged in advance by a charger 97,and a latent image is formed by the exposure on said photosensitivedrum. A latent image corresponding to the signal of a particular coloris developed in one of developing units 92-95 corresponding to saidcolor.

For example, in response to a first scanning operation of the originalimage in the color reader unit, a dot image of the yellow component ofsaid original image is exposed on the photosensitive drum 91, and isdeveloped by the yellow developing unit 92. The yellow image thusobtained is transferred onto a sheet wound on a transfer drum 96, bymeans of a transfer charger 98 at the contact point of thephotosensitive drum 91 and the transfer drum 96, whereby a yellow tonerimage is formed on said sheet. The same process is repeated for magenta,cyan and black colors, and these color images are superposed on saidsheet to obtain a four-color toner image.

As the characteristics of the developer in the developing units isdependent on humidity, the image densities under the same image formingconditions vary as a function of humidity as shown in FIG. 9. Also FIG.10 shows the image density as a function of the surface potential of thephotosensitive drum, in which humidity is taken as a parameter.

Consequently the required target potential for a fixed target imagedensity D₀ is V_(C2), V_(C1) or V_(C0) respectively for a relativehumidity of 80%, 50% or 20% (in the present embodiment V_(C2) =150 V,V_(C1) =240 V and V_(C0) =300 V). It should be noticed that the imagedensity characteristic to humidity relationship shown in FIG. 9 variesfor each color, and thus the required target potential also varies foreach color.

On the other hand, in the potential characteristic of the photosensitivemember shown in FIG. 13, a contrast equal to or higher than 250Vrequires an elevated grid voltage which is unable to provide a necessaryprecision of control, so that the laser power has to be suitablyswitched. In the present embodiment, therefore, the laser power isswitched at the relative humidity of 50%, and is selected lower orhigher respectively above or below the humidity of 50%.

EXPLANATION OF TONE CONTROL CIRCUIT (FIGS. 2 AND 3)

FIG. 2 is a block diagram of the tone control circuit 21.

The 8-bit image data 11 released from the IPU 9 of the reader 100 issupplied to a buffer memory 30 in synchronization with a synchronizationsignal RHSYNC and an image clock signal RCLK from the synchronizationsignal process unit 1, and the image signal stored in the buffer memory30 is read therefrom in synchronization with signals HSYNC and CLK 32from a synchronization control unit 31. In this manner the image signalis adjusted to the difference in synchronization and speed between thereader 100 and the printer 200, and is supplied to a selector 33.

When a selection signal 34 from a CPU 25-1 of a control unit 25 selectsthe input A of the selector 33, the image signal is supplied to theaddress terminal of a look-up table RAM (LUTRAM) 38. When the CPU 25-1selects the reading mode of the RAM 38 by a control signal 36, datacorresponding to the address input are released from the RAM 38. Thereleased data are supplied to a selector 39, and further supplied to anext selector 40 by the aforementioned selection signal 34. When aselection signal 42 of the selector 40 selects the input A, said dataare supplied to a D/A converter 41 for conversion into an analog signal.

The analog image signal 41-1 thus obtained is binary encoded by a binaryencoding circuit 44, of which an example is shown in FIG. 16. Inresponse to a clock signal 51 released from the synchronization controlunit 31, triangular wave generators 44-1, 44-7, 44-13 and 44-19 generatetriangular waves, which are subjected to the regulation of gain leveland offset set by variable resistors 44-3, 44-9, 44-15, 44-21, 44-5,44-11, 44-17 and 44-23 and are compared with the analog image signal41-1 in comparators 44-6, 44-12, 44-18 and 44-24 to obtain pulse-widthmodulated signals for supply to inputs A - D of a selector 44-25.

FIG. 14-2 shows the relation between the released pulse width and theamount of laser beam emission. In order to fully utilize the linearportion of the characteristic curve corresponding to the hexadecimallevels (OO_(F) -FF_(H)) of the image signal, the aforementioned variableresistors for regulating the gain and offset levels are manuallyregulated in cooperation with an energy measuring device provided in theoptical path, in such a manner that the level OO_(H) corresponds to theinitial position of the linear portion and the level FF_(H) correspondsto a position immediately before the end of said linear portion.

However the laser current becomes different when the laser power isswitched as will be explained later. As the laser element starts lightemission above a predetermined threshold current, the amount of lightemission becomes different even with a same pulse duration as shown inFIG. 14-1. Consequently, when the laser power is varied, the linearrange of the amount of light emission as a function of the pulseduration supplied to the laser driver 22 varies depending on the laserpower, as indicated by curves (1) and (2) in FIG. 14-2, respectivelycorresponding to higher and lower laser power levels.

Therefore, in order to obtain a same image density from a same imagesignal with a varied amount of laser light emission, it becomesnecessary to regulate the pulse duration in response to the change inthe amount of light emission. This is achieved, in the presentembodiment, by employing a number of binary encoding circuitscorresponding to the number of switched levels of the laser power.Though the present embodiment employs plural digitizing circuits asshown in FIG. 16, it is also possible to use selectively plural circuitsfor controlling the gain and offset levels.

Also plural binary encoding circuits for the switched levels of thelaser power are also provided for a clock signal 3CLK 52 of a frequencydifferent from that of the clock signal 51 from the synchronizationcontrol unit 31. The selector 44-25 selects one of plural pulse-widthmodulated binary image signals according to a signal from the CPU 25-1.

The image signal pulse-width modulated by the binary encoding circuit 44is supplied, through an OR gate 45 and an AND gate 46, to the laserdriver 22.

FIG. 15 shows the details of said laser driver 22, in which an analogswitch 22-7 is controlled according to the information discriminated bythe control unit 25, thereby varying the constant current supplied tothe laser element 23, as will be explained in more detail in thefollowing.

The signal from the AND gate 46 of the tone control circuit 21 issupplied, through a buffer 22-1 in the laser driver 22, to a transistor22-2 constituting a differential circuit, of which the other transistor22-3 is used for driving the laser element 23. These transistors aregiven a constant current by a transistor 22-4. An operational amplifier22-5 receives, at the positive input terminal thereof, a signal suppliedfrom a constant voltage source 22-6, and, at the negative input terminalthereof, a voltage across a resistor R5 for detecting the current in thetransistor 22-4, and supplies the transistor 22-4 with a voltage forcausing a constant current. On the other hand, an analog switch 22-7controlled by a buffer 22-8 receiving an I/O signal from the controlunit 25, varies the voltage supplied to the positive input terminal ofthe operational amplifier 22-5, thereby varying the current supplied tothe laser element 23. In this manner the analog switch 22-7 iscontrolled according to the humidity information, thereby switching thecurrent to be supplied to the laser element.

If the point (a) is left open even momentarily at the voltage switching,the output of the operational amplifier increases to elevate the currentwithout limit, thereby eventually leading to the destruction of thesemiconductor laser. The analog switch is provided for preventing suchphenomenon.

A blanking signal 48 from the synchronization control unit 31 is usedfor turning on the laser element 23 for enabling the beam detector todetect the arrival of the beam. An inhibit signal 49 from the CPU 25-1is used for inhibiting the function of the laser element 23, therebyextending the service life thereof.

A pattern generator 50 generates a predetermined pattern for chackingthe image signal, and, it receives a transfer drum synchronizationsignal ITOP, the horizontal synchronization signal HSYNC of the printer200 and a control signal from the CPU 25-1. When said pattern signal isreleased, the CPU 25-1 shifts the selection signal 42 for the selector40 to the input B, thereby supplying the signal of the pattern generator50 to the D/A converter 41 and thus checking the image signal.

The synchronization control unit 31 releases a clock signal CLK 51 or3CLK 52 for generating a triangular wave based on a reference clocksignal from a crystal oscillator in response to an instruction from theCPU 25-1. It also receives the beam detection signal from the beamdetector 20, and releases the blanking signal 48, the horizontalsynchronization signal HSYNC of the printer 200, and the image clocksignal CLK. The binary encoding circuit 44 releases a binary encodedsignal 47 in synchronization with the CLK signal 51 or the 3CLK signal52.

FIG. 3 is a timing chart showing the timing of said beam detectionsignal and blanking signal 48.

The synchronization control unit 31 receives a clock signal, from acrystal oscillator, of a frequency larger than twice of that of theimage clock signal, and releases the signals HSYNC and CLK insynchronization with the beam detection signal and said clock signal.The blanking signal 48 is formed by a counter which is reset at the endof the beam detection signal BD and measures a period shorter than theperiod of said beam detection signal BD.

EXPLANATION OF FUNCTION OF THE READER UNIT (FIG. 4)

FIG. 4 is a flow chart showing the function of the CPU 10-1 of thecontrol unit 10 of the reader 100, and a corresponding program is storedin the ROM 10-2 shown in FIG. 1A.

When the power supply to the reader 100 is started, a step S1 executesan initial display routine, including the checking of input/outputstates, initialization of the RAM 10-3 in FIG. 1A and movement of thescanning start point. Then a step S2 detects whether the reader 100 isconnected with the printer 200. A step S3 discriminates whether a printswitch in the operation unit 16 has been actuated, and, if actuated, astep S4 sends a print-on command to the printer 200. Then a step S5awaits the entry of the signal ITOP from the printer 200, and, uponentry thereof, a step S6 initiates the scanning of the original imagewith a designated color mode and sends the image signal to the printer200.

EXPLANATION OF FUNCTION OF THE PRINTER (FIG. 5)

FIG. 5 is a flow chart of the control sequence of the control unit 25 ofthe printer 200, and a corresponding program is stored in the ROM 25-2shown in FIG. 1B.

When the power supply to the printer 200 is initiated, a step S10executes an initial routine, including the checking of input/outputstates, initialization of the RAM and removal of retentive charge on thephotosensitive drum. A step S11 then checks the connection with thereader 100, and, when said connection is confirmed, a step S12discriminates whether the heater of the fixing unit has been warmed upto a predetermined temperature. Upon completion of the warming-up, astep S13 discriminates whether a print command has been sent from thereader 100. In response to a print command, a step S14 (S14-1-S14-4)executes a process PGON to be explained later, respectively for theclock signals used for generating the triangular wave and forcontrolling the laser power.

A step S15 calculates the data to be stored in the LUTRAM 38 accordingto the humidity data and the character/photograph information (data forselecting the clock signal CLK or 3CLK) from the reader, based on theresult of the step S14 as will be explained later. The clock signal CLKor 3CLK is selected respectively for the character information or thephotograph information. The calculated data are stored in the LUTRAM 38in a step S16, by selecting the input terminal B of the selector 33 bythe selection signal 34 and connecting a data bus 36 of the CPU 25-1through the selector 39 to the data input terminal of the LUTRAM 38. TheCPU 25-1 releases the address of the LUTRAM 38 to an address bus 35 andthe data to be stored to a data but 37, and the storage into the LUTRAM38 is conducted by the entry of writing pulses in response to thecontrol signal 36.

Then a step S17 discriminates whether the storage in the LUTRAM 38 hasbeen completed, and, if completed, a step S18 sends the signal ITOP tothe reader 100. In response to said signal, in the flow chart shown inFIG. 4, the sequence proceeds from the step S5 to S6. Then a step S19sets a designated color mode, and the tone control is executed byswitching the address of the LUTRAM 38 for each color. A step S20executes a printing operation of the designated color. Upon completionof a designation color mode of color image formation, the sequencereturns to the step S11.

EXPLANATION OF PGON PROCESS (FIG. 6 AND 7)

The PGON processes in the steps S14-1 to S14-4 are summarized in FIG. 6as they are the same except for the laser power and the clock signalused for generating the triangular wave.

FIG. 6 is a flow chart of the PGON process in the step S14 in FIG. 5,for activating the pattern generator 50 to release a predeterminedpattern and reading the surface potential of the photosensitive drum.

At first a step S30 causes the selection signal 42 to select the inputterminal B of the selector 40, for supplying the signal of the patterngenerator 50 to the D/A converter 41. Then a step S31 causes thepotential measuring unit 27 to measure the potential which is generatedon the photosensitive drum 29 by a laser beam emitted in response to asignal, for example "00", from the pattern generator 50. The binarydigitizing circuit 44 is so adjusted in advance that the comparators44-6, 44-12, 44-18 and 44-24 release a limit pulse enough for inducinglight emission from the laser element in response to a "0" input signalto the D/A converter 41. Thus the photosensitive member 29 is uniformlyirradiated by the laser driver 22 and laser element 23.

Also the binary encoding circuit 44 is so adjusted in advance that thelaser element 23 emits light with a period shorter than the period ofthe triangular wave, thereby exactly reproducing dots when the patterngenerator 50 releases a hexadecimal signal "FF" in the step S31, and thepotential corresponding to said signal "FF" is read in the same manner.

A step S32 determines the target surface potential V_(CO) from FIG. 10in order to provide a predetermined image density in response to thedetected humidity, and discriminates whether the difference of themeasured potentials V₀₀ and V_(FF) respectively corresponding to thesignals "00" and "FF" from the pattern generator 50 is equal to apredetermined value. If not, the sequence proceeds to a step S33 forvarying the high voltage of the charger 97 shown in FIG. 1, and thesequence returns to the step S31 for repeating the procedure.

On the other hand, if said difference in the step S32 is equal toV_(C0), the sequence proceeds to a step S34 for activating the patterngenerator 50, whereupon the pattern generator 50 starts to function asan m-bit counter for counting the HSYNC signal in synchronization withthe ITOP signal, and releases signals in succession, by dividing thesignals "00" to "FF" into a predetermined number m of levels. The signalthus obtained is supplied through the selector 40 to the D/A converter41 for obtaining an analog signal for driving the laser element 23.Steps 35 and 36 read the potential of the photosensitive member 29varying in m levels in response to said analog signal, and store saidpotential in succession corresponding to the output signal of thepattern generator 50. In the present embodiment said number m is takenas 16.

FIG. 7 shows the relation between the input signal of the D/A converter41 and the voltage measured by the potential measuring unit 27.

The photosensitive member 29 is charged to a negative potential, so thatthe potential is elevated by the irradiation with a laser beam, andnegatively charged toner is correspondingly deposited. In FIG. 7, V_(DD)indicates a charge level when the laser is not activated, and V_(L)indicates a charge level when the laser is fully activated.

In the present embodiment, the PGON process is executed, prior to everycopying sequence, for all the laser power levels and the clock signalfor generating the triangular wave, but said process may be conducted ata predetermined interval with a suitable timer, or after a predeterminednumber of copying operations. Also it may be conducted for a selectedlaser power level and a selected clock signal.

It is furthermore possible to employ the exclusive sequence for the PGONprocess to store the measured values, and to prepare a look-up table bymeans of the stored data at the ordinary copying sequence.

PREPARATION OF LOOK-UP TABLE (STEP 21) (FIG. 12)

FIG. 12 shows the relationship between the input image signal and theoutput image density wherein;

1st quadrant indicates the output density D as a function of the inputlevel e;

2nd quadrant indicates the relation (LUT) between the conversion level Eand the input level e;

3rd quadrant indicates the relation (EV curve) between the conversionlevel E and the potential V measured by the potential sensor; and

4th quadrant indicates the relation (VD curve) between the outputdensity D and the measured potential V; wherein:

    V=(value measured by potential sensor-V.sub.FF)/(V.sub.00 -V.sub.FF)

    D=(density/maximum density)×"FF"

V₀₀ : potential measured by potential sensor in response to a signal"00"

V_(FF) : potential measured by potential sensor in response to a signal"FF".

The VD curve is selected from plural curves stored in advance in the ROM25-2 according to the developer, laser power levels and the clocksignals for generating triangular wave for use in the binary encodingcircuit 44 in FIG. 2 (clock signals CLK 51, 3CLK 52).

Since the EV curve is almost linear, the PG process for potentialmeasurement (step S34 in FIG. 6) is conducted by storing a tablecorresponding to the selected VD-curve (table with inverted x- andy-axes of the VD-curve) in the LUTRAM 38, and utilizing the dataconverted by the table in the LUTRAM 38. Said data stored in the LUTRAM38 may also be stored in advance in a ROM.

The preparation of the look-up table is conducted in such a manner thata conversion level Ei is obtained corresponding to the input level eiand the density level Di, in order that the output density D varieslinearly as a function of the input level e, wherein the output ei ofthe pattern generator provides a measured potential Vi corresponding toa density Di according to the VD curve. As the output of the patterngenerator is varied in 16 levels in the present embodiment, there areprepared 16 LUT data, and said data are completed from "00" to "FF" byapproximation with folded lines.

In the foregoing embodiment the look-up table is formed in a RAM, but itis also possible to store plural data groups in a ROM and to select asuitable group according to the result of calculation by the CPU.

As explained before, the foregoing embodiment allows a stable image tobe obtained by maintaining a constant relationship between the potentialon the photosensitive member and the image signal and also taking thecharacteristics of the developer into consideration. Also in case of acolor image, it allows the prevention of fluctuation in colors, therebyproviding an image with constant color.

Though the foregoing description has been directed to a color imageforming apparatus in which a laser and an electrophotographic processare combined, the present invention is not limited to such embodiments.In fact the present invention it is subject to various modificationswithin the scope and spirit of the appended claims.

What is claimed is:
 1. An image forming apparatus comprising:imageforming means for forming an image on a recording member, said imageforming means including exposure means for exposing said recordingmember; first detector means for detecting a surface condition of saidrecording member; second detector means for detecting environmentalconditions in the apparatus; and control means for controlling anoperational condition of said image forming means in accordance withoutputs of said first and second detector means, said control meansselecting one reference value from among a plurality of predetermineddifferent reference values in accordance with an output from said seconddetector means, the operational condition of said image forming meansbeing determined in accordance with (1) the selected reference value,and (2) an output of said first detector means, the amount of light ofsaid exposure means being selected in accordance with the output of saidsecond detector means.
 2. An image forming apparatus according to claim1, wherein said image forming means comprises charger means for chargingthe recording member, and wherein said first detector means is adaptedto detect the surface potential of said recording member.
 3. An imageforming apparatus according to claim 2, wherein said control means isadapted to control the operational condition of said image forming meansso as to obtain a contrast potential according to said reference value.4. An image forming apparatus according to claim 3, wherein said controlmeans is adapted to control an operational condition of said chargermeans.
 5. An image forming apparatus according to claim 1, wherein saidsecond detector means is adapted to detect the humidity in saidapparatus.
 6. An image forming apparatus according to claim 1, whereinsaid control means is adapted to switch the amount of light of saidexposure means when the selected reference value exceeds a predeterminedvalue.
 7. An image forming apparatus according to claim 6, wherein saidexposure means comprises a laser element.
 8. An image forming apparatusaccording to claim 1, wherein said first detector means detects thesurface condition in an area formed on the recording member when saidexposure means is operated with an amount of light selected inaccordance with the output of said second detector means, and whereinsaid control means controls the operational condition of said imageforming means in accordance with the thus-detected value of said firstdetector means and the selected reference value.
 9. An image formingapparatus according to claim 8, wherein said image forming meanscomprises charger means for charging the recording member, and whereinsaid surface condition comprises a surface potential of said recordingmember.
 10. An image forming apparatus according to claim 9, whereinsaid first detector means is adapted to detect surface potentials of anexposure area and non-exposure area on said recording member.
 11. Animage forming apparatus according to claim 10, wherein said referencevalue comprises a target contrast potential, and wherein said controlmeans controls an operational condition of said charger means such thatdifference between the detected surface potential of the exposure areaand the detected surface potential of the non-exposure area is equal tosaid reference value.